In China, its age 7 (no offense guys....its in good humor!!!)...
In the US see the query/answer below from Murthy.com
Chat User : I am waiting for my AOS. It has been pending for the last 2 years. Can my 17-year- old dependent son apply for his own EAD card? Does this cause any problems?
Attorney Murthy : The law allows a child over the age of 16 years to apply for the EAD to be able to work legally during the summer vacations, etc. This should not cause any problems for the child. Since the child has already filed the I-485, the child's age will likely be frozen in time under the Child Status Protection Act if the family filed the I-485 within one year of the I-140 approval so that, even if the child reaches 21 years of age, the child should be entitled to adjust status within the U.S. without being left out by the system.